The present invention relates generally to apparatuses and method for providing tunable filter for optical fiber signal communication systems. More particularly, this invention relates to new configurations and methods for implementing a far off-axis, large Bragg""s deflection angle, non-collinear Acousto-optical tunable filter having a miniaturized size and very narrow bandwidth suitable for DWDM applications with 25 Ghz, 50 Ghz and 100 Ghz channel spacing adapting no moving parts or thermal tuning to achieve high speed reliable tuning.
Contemporary means of tunable filters for the optical telecommunications market use mechanical actuators such as piezzo electric devices, MEMS, step motors along with high-density gratings to achieve narrow bandwidth wavelength selection and tuning. Still others use thermal means on a grating device, which is either by direct heating/cooling or by indirect (such as current) of heating/cooling, to change the spacing of a thermally sensitive grating to achieve wavelength tuning. These methods, although feasible and commercially available, are very slow in tuning speed, e.g., tuning speed in the neighborhood of hundreds of milliseconds or seconds. Stability often becomes a problem too caused either by the slow response of sensing the temperature for thermally tuning the devices or caused by the high susceptibility to vibration and shock when a moving part is implemented for tuning the devices.
In contrast, tuning technologies implemented with Acousto-optical tunable filter (AOTF) using the birefringent crystals would have a higher likelihood to resolve the above-mentioned difficulties. However, tuning with conventional AOTF techniques is also faced with limitations of size and bandwidth. A typical AOTF with collinear configuration with TeO2 in share mode as that disclosed by Chang, et al as will be discussed below can achieve nanometer filter bandwidth but with very big sizes, e.g., a 2 nm bandwidth collinear AOTF would require a crystal 30 mm length. Similar constraints exist for non-collinear configuration with TeO2 in share mode and with low RF frequency applications also disclosed by Chang, et al.
Typical AOTF applying birefringent crystals are produced with specific configurations, namely, when a crystal is cut, the PZT electrodes platting surface is cut, instead of perpendicular to, but a mall angle (Qa) from the crystals [110] axis. All crystals have three principle axis [100], [010] and [001] as designated in [x y z] axes for optical interactions. For homogeneous crystals, all optical properties are the same along all three axes. For inhomogeneous crystals, optical properties along different principle axis may behave differently as that of birefringent crystals. However, acoustic waves may not necessarily travel along the optical axis. There are two types of acoustic traveling waves: Longitudinal and Share. The Longitudinal wave is a compression wave and the Share wave is simply to the light wave oscillating up/down while traveling horizontally for example. For xe2x80x9con-axisxe2x80x9d share wave TeO2, it travels along [110], in Z plane and along the diagonal of X and Y-axes. For xe2x80x9cOff-axisxe2x80x9d share wave, it typically refers to what FIG. 1A depicts, a small angle off [110] and Z plane. For angles larger then 10 degrees, it is often referred to as xe2x80x9cfar off axis designsxe2x80x9d. The off axis combination as shown may eliminate the need for tilted crystals. It may improve optical degeneracy, which happens often with symmetrical designs as that of the configuration shown in FIG. 1A as will be further discussed below wherein the crystal can be used with either surfaces functioning as the xe2x80x9cfrontxe2x80x9d surface.
When tuning with a radio frequency (RF) signal with a high frequency, the acoustic wave inside the crystal decays rapidly making the birefringent crystals, e.g., TeO2, almost unusable for RF signal with frequency higher than 200 MHz. For that reason, almost all non-collinear share-mode TeO2 designs are implemented with low RF frequency lower than 100 MHz. With an optical wavelength of 1.55 xcexcm, the RF frequency is in a range of 20-50 MHz and the filter bandwidth is very wide. In terms of grating effect, its grating line density is too low to be, useful for use on tunable filters in a telecommunication system, in particular for a divisional wavelength demultiplexing (DWDM) system, with which the passband requirement is less than 0.2 nm. One way to narrow the filter bandwidth is to increase the crystal length along optical path direction, which results in a very long crystal. Typically for 2 nm wide Acousto-optical tunable filter (AOTF), the crystal length is more then 30 mm long. For modern telecommunication systems that required miniaturized devices, conventional technologies of applying acoustic-optical tuning techniques are not able to provide effective solutions for making a useful AOTF suitable for tunable laser implementations.
Chang disclosed in several United States patents, e.g., U.S. Pat. Nos. 5,329,397, 4,720,177, 4,582,397, 4,343,503, and 4,052,121, techniques of using collinear and non-collinear electrically tunable Acousto-optical filters. These filters are implemented with interactions between the acoustic and optical waves in the acoustically an-isotropic and optically birefringent crystals. FIGS. 1A and 1B are diagrams for showing the Optical Refractive Index Ellipsoid in a birefringent TeO2 crystal cut along particular axis and as a result shown in the figures that the X axis is along the diagonal of two other principle axes, the Y axis is shown as the principle axis. The symbol K designates wave vectors, e.g., the direction of incident light is projected along a direction shown by Ki and the direction of projection of the diffracted wave is shown in a direction of Kd and the direction of the acoustic wave is along a direction shown as Ka. For a share wave, the direction along [110] is the propagation direction. Therefore, in FIG. 1, the acoustic wave direction Ka is slightly non-parallel to [110] indicating the xe2x80x9coff-axisxe2x80x9d applications. The length of Ka measures the RF frequency (fa). In FIG. 1A, according to the prior art AO non-collinear filter of Chang, with TeO2 in Share-mode, in which all three waves are traveling along different directions satisfying Bragg""s law of diffraction with Ki and Kd are on the same side of the quadrant thus resulting a low value of Ka, i.e., low RF frequency. At high frequency, the acoustic wave inside the crystal decays rapidly, especially in the rang of visible wavelength, the AO filter is almost unusable for an RF frequency fa greater than 200 Mhz. For this reason, almost all non-collinear Share-mode TeO2 Acousto-optical filters are with implemented with low RF frequency  less than 100 Mhz. With 1.55 um optical wavelength, the RF frequency fa is in the range of 20-50 Mhz and the filter bandwidth becomes very wide. Hence, the grating line density is too low for application as tunable filters in a telecommunication system with a passband less than 0.2 nm. In order to narrow down the filter bandwidth, the length of the crystal has to increase along the direction of the optical path thus preventing further miniaturization of the AO filters implemented with such a technologies. FIG. 1B shows a similar configuration with collinear design with TeO2 in Share-mode, in which all three waves are traveling along the same direction also satisfying Bragg""s law of diffraction. Again, for an optical wavelength of 1.55 um, the RF frequency (fa) is about 23 Mhz and the collinear filters encounter similar technical limitations as that discussed above for a non-collinear AO filters. Since the fiber optical signal transmissions are now more commonly implemented in the telecommunication and network systems, and as the tunable lasers using the Acousto-optical tunable filters are key and important devices for such systems, there is an ever-urgent demand to resolve these limitations and difficulties. FIG. 1C illustrates a typical implementation on a birefringent crystal for an Acousto-optical deflectors. The incident wave propagates perpendicular to or almost perpendicular with a small incidence angle relative to the acoustic wave. The deflected light that is diffracted to a different propagating direction can be adjusted to propagate along different tunable angles by changing the frequency of the acoustic wave thus creating a scanning effect. This Acousto-optical configuration is often used as scanner or deflector. As will be further discussed below in FIGS. 4A and 4B, the frequency range as shown in FIG. 1C represents a zone where the angle of incidence and angle of deflection angle near zero relative to the [001] axis.
More specifically, in fiber telecommunications, tunable components including tunable lasers, tunable filter, tunable attenuators, etc. are essential to provide system reconfiguration and reprogramming and the key parameters in optical networks are the speed, range, stability, and flexibility of wavelength tuning. Particularly, as digital video, audio and wide varieties of digital data and signals are transmitted via broadband networks, the lack of flexibility in network management becomes an ever-increasing headache for network managers among carriers. As optical network deployment approaches saturation and becomes ever so complicated and expensive, re-deployment and re-configuration become necessary. Therefore, flexible/tunable optical components become essential for next generation optical telecomm equipment. Different network systems implemented with tunable components are being developed currently or in the near future at major equipment makers. Furthermore, tunable filters, besides its use in a tunable laser deployed network, it finds its use also in the OADM and similar configurable switching networks.
For these reasons, the Acousto-optical tunable filters even can be tuned with high tuning speed without moving parts and thus are more reliable, are still of limited usefulness for application to the optical fiber communications networks. Therefore, a need still exists in the art of optical fiber system and component manufacturing and design, particularly those related to AOTF for tunable laser applications, to provide new and improved system and component configurations and designs to overcome the above-mentioned technical difficulties and limitations.
It is therefore an object of the present invention to provide a new and improved Acousto-optical tunable filter (AOTF) suitable for application as a tunable filter for the optical telecommunication industry that is economical, reliable, robust and with superior optical performances such that the above mentioned limitations and difficulties can be resolved. Specifically, it is an object of this invention to provide a tunable filter implemented with an Acousto-optical tunable filter that can be tuned with an RF signal of high frequency such that the AO filter can be provided with miniaturized size. With the implementation of the AOTF, the tunable filter is provided with no moving parts for the fiber networks that offers broadband tunability, high output power, narrow filtering line-width and highly reliable. Furthermore, the method of tuning as disclosed in this invention is non-thermal and non-mechanical such that the tuning speed is in the sub-microsecond range.
Furthermore, it is the object of this invention to provide an AOTF with miniaturized size that has high channel density and high tuning speed with a configuration that is convenient to manufacture and can be produced economically at a relatively low cost. Specifically, the manufacturing process can be performed with highly automated processes as that applied in the electronic industries for manufacturing the integrated circuits and electronic package and assembly processes. The tunable filters now implemented with the improved and miniaturized AOTF of this invention can be produced with competitive price and can be practically implemented in wide ranges of optical fiber networks for telecommunication applications.
Briefly, in a preferred embodiment, the present invention discloses an Acousto-optical tunable filter (AOTF) implemented in a birefringent crystal tunable by an RF signal having a frequency substantially greater than 100 MHz with a filter bandwidth in a sub-nanometer range. In a preferred embodiment, the AOTF includes a birefringent crystal provided for a far off-axis optical transmission with which its frequency is tunable (selectable) by applying an acoustic wave projected substantially and close to perpendicular to the optical transmission. In another preferred embodiment, the AOTF wherein the on-axis optical transmission is with an angle of incidence along a [001] lattice axis in the birefringent crystal and the tuning acoustic wave is projected substantially along a [110] lattice axis.
The AOTF of this invention is suitable for implementation as a passband filter of about 0.2 nm external cavity tunable laser. This external cavity tunable laser includes an external cavity tunable laser that includes a frequency-tuning device configured as an Acousto-optical cell including a first and a second Acousto-optical diffraction means having a narrow-band optical filtering Bragg grating. In a preferred embodiment, the first Acousto-optical diffraction means includes a first Acousto-optical crystal and the second Acousto-optical diffraction means includes a second Acousto-optical crystal. The external cavity tunable laser of further includes a reflection mirror driven by a PZT assembly to reflect a beam projected from the Acousto-optical cell back to transmit therethrough again. The external cavity tunable laser further includes a first electrode connected to the first Acousto-optical diffraction means and a second electrode connected to the second Acousto-optical diffraction means. The first and second Acousto-optical diffraction means having diffraction phase gratings for intra-cavity narrow-band wavelength filtering and the first electrode is connected to an RF signal for tuning a central frequency of the narrow band Bragg grating the second electrode is connected to a second electric source to provide a second order filtering for compensating a wavelength shift.
In a preferred embodiment, this invention discloses a frequency-tuning device disposed in an Acousto-optical birefringent crystal. The frequency-tuning device includes a means for applying an acoustic frequency onto the frequency-tuning device in an acoustic frequency range to effect a divergent projection between an optical incident angle and a diffracted angle relative to an optical axis as the acoustic frequency is increased. In a preferred embodiment, the means for applying an acoustic frequency onto the frequency tuning device in the acoustic frequency range to diffract a far-off-axis incident optical signal relative to the optical axis to a diffracted optical signal along a corresponding divergent diffracted angle. In another preferred embodiment, the means for applying an acoustic frequency onto the frequency-tuning device further comprising an RF tuning device for applying an RF signal having a frequency around 300 MHz. In another preferred embodiment, the frequency-tuning device disposed in a TeO2 crystal. In another preferred embodiment, the optical incident signal projected substantially along a [001] crystal axis in a TeO2 crystal. In another preferred embodiment, the optical incident signal projected substantially along a [001] crystal axis in a TeO2 crystal and the diffracted signal projected along a negative diffracted angle relative to the [001] crystal axis in the TeO2 crystal. In anther preferred embodiment, the means for applying an acoustic frequency onto the frequency tuning device further comprising an RF tuning device for applying an RF signal suitable for constructing a sub-nanometer band-pass filter. As oppose to other means of tunable filters in optical telecomm applications, the present invention adapts no moving parts, it is non-thermal and achieves high speed with broad tuning range while providing long-term stable reliable operations under severe operational environments.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures.